Auto/Truck 14V Electrical
Systems Testing Tutorial
Systems Testing Flip-Chart
Shows you how!
This free tutorial
is provided on testing a vehicle's primary 14V electrical system.
A vehicle's primary electrical system consists of the battery,
the generator (formerly called an alternator) and the engine
and accesssory grounds. With FIRST THINGS FIRSTTM you can test the battery, engine and
accessory grounds and perform a thorough charging voltage analysis
in 5-8 minutes with a little practice using our FIRST THINGS
FIRST THINGS FIRSTTM begins with electrical tests before the
engine is cranked, when the vehicle is first started and electrical
testing continues through engine warm-up to ensure that there
are no primary electrical system problems in the primary electrical
system that will affect vehicle performance and driveability.
The complete step-by-step electrical system test procedure using
only your DMM is contained in our electrical troubleshooting
flip-chart we call FIRST THINGS FIRSTTM because it's the FIRST
thing electricaly to do.
flip-chart version is available for the professional technician
to purchase for $70.00 plus S/H.
Read the following tutorial of FIRST THINGS FIRSTTM to get an idea of the
effectiveness of using FIRST THINGS FIRSTTM without the need for
extensive electrical training to use the flip-chart.
Please feel free to print out your own copy of this tutorial.
Of A Vehicle's Primary Electrical System
The generic block diagram of a car or truck's
primary 14V electrical system is shown in Figure 1 and is the
schematic diagram used in the flip-chart FIRST THINGS FIRSTTM. The illustration consists of a Battery,
and an Generator (Alternator) which is referred to by
the name "generator" (an old term coming back into
use since 1996) and the vehicle's Primary Ground System
consisting of an Accessory Ground and an Engine Ground.
To become more familiar with a vehicle's primary electrical system,
trace the wires in Figure 1 between all components to see how
they connect together.
The vehicle's battery stores electrical
energy in chemical form and delivers large amounts of electrical
energy (current) on demand to crank the engine. Identify both
battery posts on the battery in Figure 1. One post is marked
"+" (positive) or "hot" side and has
a red or black battery cable connected to it. The Junction
Block is a voltage side common connection point used on many
vehicles. It is located in the engine compartment near the battery.
The negative battery post is marked "-" (minus)
and is called "ground.". The negative battery cable
is usually a two wire black cable. The smaller diameter cable
is connected to the sheet metal for the Accessory Ground.
The larger diameter wire is connected to the engine block for
the Engine Ground.
The Generator (Alternator)
The generator (alternator) produces
a charging voltage to electrically power the vehicle and recharge
the battery as long as the engine is running and the alternator
is performing properly. The positive output terminal of the alternator,
called the "B+" terminal, is a large insulated
terminal on the rear or side of the generator (alternator) housing
with a heavy gauge wire connecting back to the battery's positive
(+) post through the Junction Block. The generator (alternator)
housing is the generator (alternator)'s negative (-) terminal
Trace both the positive and negative battery cables from the
battery posts to their connection points in the vehicle. Battery
"+" goes to generator (alternator) "+"
(the B+ terminal) through the wiring harness. Battery
"-" goes to generator (alternator) "-"
through the engine ground and the engine block. The purpose of
the voltage regulator is to excite the generator (alternator)
and prevent the charging voltage from rising above a preset limit.
The voltage regulator may be mounted external to the alternator
and bolted to the vehicle's sheet metal with wires connecting
to the generator (alternator) or it may be mounted inside the
generator (alternator). On many newer vehicles the voltage regulator
may be computer controlled by a vehicle computer for more precise
control of the charging voltage. This flip-chart may be used
regardless of where the voltage regulator is mounted. Testing
the primary electrical system using FIRST THINGS FIRSTTM requires only
a digital voltmeter (DMM).
Do This On A Vehicle First
Look under the hood of the car or truck to
be tested and make sure all connections and wires around the
battery and generator (alternator) are tight, in good condition
with no corrosion or damaged insulation. Identify both "+"
and "-" battery posts and visually inspect for corrosion
around the battery cable terminals where corrosion is normally
a big problem. Tug on the battery cables to make sure that corrosion
has not caused the cable ends to form a bad connection where
the cable enters the battery cable terminal end. Next, locate
the generator (alternator) and the B+ terminal. Inspect the wire
at the generator (alternator)'s B+ terminal to make sure it is
clean and making a tight connection.
What You Need To Know About
Digital Multimeters (DMMs)
There are two types of DMMs in use.
(1) Autoranging DMM automatically selects the proper range
for a DC Voltage being measured. (2) Manually Selected DMM requires
the technician to manually select the correct voltage range using
the main function knob.
A block diagram of an Autoranging
DMM is shown in (1) of Figure 2. Note the symbol for the DC Voltage
function. It is a small solid line above a small dotted line
followed by a "V". This means "DC Volts."
By selecting DC Volts, an Autoranging DMM automatically sets
to the lowest milli-volt (mV) DC Voltage range, usually 200,
300, or 400 mV depending on the DMM brand. All DMMs display 00.0
or 000.0 when on the milli-volt range so 100 mV (or 0.10V) would
read as 100.0 (mV). When an Autoranging DMM senses the test voltage
is above the value of the milli-volt range, the DMM automatically
steps up to the next higher voltage range which is 2.00, 3.00
or 4.00 V DC range, depending on the DMM brand. All DMMs display
.000 on the 2/3/4 volt range so 1.5 volts would read as 1.500
When an Autoranging DMM senses the test
voltage is above the value of the 2/3/4 Volt range, the DMM automatically
steps up to the next higher voltage range which is the 20.00,
30.00 or 40.00 V DC range, depending on the DMM brand. This is
the highest range the DMM must "step-up" to since cars
and trucks run on 12-15 volt systems and no voltage greater than
20 volts is measured. All DMMs display 0.00 on the 20/30/40 Volt
range so 14.25 volts would read as 14.25 (V).
If 145 mV is measured, the DMM drops back down to the mV range
and reads 145.0 mV. As an Autoranging DMM jumps around between
the mV, 2/3/4V and 20/30/40V ranges, it can get very confusing
and cause reading errors which lead to misdiagnosis when troubleshooting.
If using an Autoranging DMM here's how to make it easy and avoid
the confusion caused by the DMM automatically changing voltage
ranges between test steps.
Manually select the 20V, 30V or 40V range which ever your
DMM has and leave it there while performing these FIRST THINGS
FIRST electrical tests. Simply press the RANGE BUTTON until
the readout first says 0.00. This is the 20, 30 or 40 Volt range
and a ten millivolt reading is 0.01 while a 14 volt reading is
14.00. The 20V, 30V or 40V range is the best DMM range for performing
the electrical tests as explained in FIRST THINGS FIRST.
Manually Selectable DMM
A manually selected DMM, is represented
in (2) of Figure 2 and is easier to use. They have a main function
knob in the center of the DMM face plate that permits a technician
to manually select the desired range. Manually select the 20/30/40
Volt ranges. The readout should read 0.00. The DMM then
stays on the 20/30/40 volt range for all tests. Leave the knob
set to 20/30/40 volt range for all the test steps. A comparison
of voltage readings for both types of DMM is shown in Figure
Use Either Type DMM (Autoranging
or Manually Selected)
Select the 20, 30, or 40 DC voltage range
on the DMM. This is when the DMM first indicates 0.00. Leave
the DMM on this range for all tests. DO NOT ALLOW THE DMM
TO AUTO-RANGE DURING MEASUREMENTS AS IT COULD CAUSE CONFUSION
INTERPRETING READINGS WHICH WILL VARY BETWEEN VOLTS AND mV.
Place the DMM test leads into the vehicle as shown in the illustration
for each test step and follow the written instructions on the
left side of the page for that step. Compare your DMM reading
with the diagram of each test. The text discusses what to do
if an incorrect reading is obtained. Follow directions given
on each page for repair procedures. Step 1 is provided
so you can sample the format.
How To Use The Test Procedure
Each of the 15 test steps are explained
and illustrated on their own page. On the left side of the page
are instructions on how to perform each test step. On the right
side of the page is a picture of the primary electrical system
showing how the DMM test leads placed into the vehicle to make
each measurement. Follow the illustration of the DMM test leads
when connecting your DMM to the vehicle. The DMM also shows the
approximate reading you should find in the vehicle. Compare your
DMM reading with the reading in the illustration. If the reading
can be a little higher or a little lower than what is shown in
the illustration, the text will explain the acceptable upper
and lower limits of a reading needed to pass or fail a test step
and what should be done to correct a problem.
Connect your DMM Test Leads to the DMM
as shown in the illustration to the right. The Red (+) DMM Test
Lead connects to the V/Ohm jack. The Black (-) DMM Test Lead
connects to the COM (Common) jack. Here is a sample of how FIRST THINGS FIRSTTM guides you through each step with these paragraph
Title of the measurement
Do This . . . .
Expect This . . . .
Tips . . . .
Here's a sample of FIRST THINGS FIRSTTM Step 1
. . . . . . .
Step 1: Measure
Battery Open Circuit Voltage
This paragraph explains the exact
procedure to follow to perform the test step. For example, it
will say "with the ignition key OFF, measure the battery
terminal post voltage." The illustration also shows how
the test leads are connected into the vehicle. Follow the illustration.
This paragraph explains the expected
result so it can be determined that the vehicle passes or fails
a test. Also included are the range of readings that should be
expected in this test. If the reading is a little lower or a
little higher, the exact numbers are given in the text for comparison
with what is found in the vehicle to determine pass or fail.
The paragraph "Expect This:" suggests items to check if the reading is
incorrect (too high or too low) and the vehicle fails a test
step. Also listed are items to double check to avoid common mistakes
made when performing each test step. When the vehicle passes
each test step, go to the next page and perform the next step.
Do not move to the next step until the correct reading is obtained
from either the vehicle passing the test or as a result of a
suggested repair procedure correcting a problem.
Tips: Some of the test steps offer tips
on what to check next if a DMM reading on a particular test step
is too high or too low.
remaining electrical system test Steps 2-15 are found in FIRST THINGS FIRST. Order below!
When you first receive your copy
READ THROUGH All 15 STEPS FIRST to get familiar with the tests.
Then begin performing FIRST THINGS FIRSTTM.
Go slow the first time you perform the electrical tests to thoroughly
read through and understand what you are doing. Then practice
all 15 test steps and record readings on the page "Test
Results Record" provided on the last page of each flip-chart.
Feel free to make extra copies of the Test Results page to record
Show or give the copy to your customer with the readings found
during the tests.
With a little practice most technicians can perform the electrical
tests in FIRST
in 5-8 minutes.
This tutorial reviewed basic concepts
of a vehicle's primary electrical system and appropriate electrical
system test procedures with an introduction to FIRST
We trust you will find this tutorial helpful.
To order a laminated copy of FIRST THINGS FIRSTTM simply click on the red "ADD-TO-CART
button, $70.00 plus s/h. We'll include information about our
other training programs you can study at home.
you like more training in vehicle electrical-electronics troubleshooting?
We have the training for you, either home study or live training
presented on-site by Vince Fischelli.
techs who like training material in printed form look at our
60 lesson home-study course in vehicle electrical - electronics
here. Now also on CDROM.
To those techs who prefer more of a hands-on approach to electrical
training look at our hands-on home-study vehicle
electrical - electronics troubleshooting training program click here.
If you would
like to attend live electrical - electronics troubleshooting
training workshops in Dallas personally taught by Vince Fischelli click here for information.
a SHOP OWNER
page is just for you.
If you're a Tech School Teacher click
page is just for you.
Return to the
FIRST THINGS FIRST page
Lane, Garland, Texas 75042-5419
Return to Top